In an automatic power transmission for an automotive vehicle, the gear ratios in the forward and reverse drive ranges of the transmission are selected by selectively actuating fluid operated frictional units including, for example, a high-and-reverse clutch, a forward drive clutch, a brake band and a low-and-reverse brake which are all operated at fluid pressure delivered from a hydraulic control system. In a known power transmission using these frictional unit, the forward drive clutch in particular is maintained coupled throughout the conditions in which an automatic forward drive range is established in the transmission. When forward drive clutch alone is permitted to be operative, the first or "low" gear ratio in the automatic forward drive range is established in the transmission mechanism. If the brake band is applied with the forward drive clutch maintained in the coupled condition, then a shift is made in the transmission mechanism from the first gear ratio to the second gear ratio in the automatic forward drive range. If, furthermore, the brake band is released and the high-and-reverse clutch in turn is made operative with the forward drive clutch held coupled, there is produced in the transmission mechanism an upshift from the second gear ratio to the third or "high" gear ratio in the automatic forward drive range. When, on the other hand, both of the high-and-reverse clutch and the low-and-reverse brake are made operative, a reverse drive gear position is obtained in the transmission mechanism. The high-and-reverse clutch is thus put into operation when either the third gear ratio in the automatic forward drive range or the reverse drive gear position is to be selected in the transmission mechanism.
The hydraulic control system for use in an automatic power transmission of this narure includes shift valves responsive to the road speeds of a vehicle and control upshifting and downshifting between the automatic forward drive range of the transmission system depending upon the vehicle speed. When the transmission gear shift valve is held in the automatic forward drive range position by the vehicle driver, the forward drive clutch is first coupled without respect to the vehicle speed. If the vehicle speed reaches a certain relatively high level, one of the shift valves incorporated in the hydraulic control system causes fluid pressure to be directed to the fluid operated servo unit for the brake band for effecting an upshift from the first gear ratio to the second gear ratio in the automatic forward drive range. If the vehicle speed is increased to a certain still higher level thereafter, another shift valve in the hydraulic control system is conditioned to direct fluid pressure to the high-and-reverse clutch for making an upshift from the second gear ratio to the third gear ratio in the automatic forward drive range. If the driving torque delivered from the engine becomes deficient for the vehicle speed being produced or the vehicle speed is reduced to a certain level under conditions in which the vehicle is being driven with the third forward drive gear ratio established in the power transmission, then one of the shift valve is conditioned to discharge fluid from the fluid chamber of the high-and-reverse clutch and the brake-apply fluid chamber of the servo unit for the brake band so that a downshift is automatically made between the second and third gear ratios in the automatic forward drive range.
When the brake band is to be released during shifting from the second gear ratio to the third gear ratio, a fluid pressure is introduced into the brake-release fluid chamber of the band servo unit. Since a line pressure is maintained in the brake-apply chamber of the band servo unit when the second and third forward drive gear ratios are in play, the fluid pressure thus admitted into the brake release fluid chamber of the band servo unit is opposed by the line pressure in the brake-apply fluid and can be increased to the level of the line pressure at a relatively high rate. In the case of the high-and-reverse clutch, however, the fluid pressure introduced into the fluid chamber of the clutch is initially consumed in moving the piston in the fluid chamber so that there is produced a dynamic flow in the fluid circuit through which the fluid pressure is directed to the fluid chamber of the high-and-reverse clutch. The dynamic flow of the fluid is subjected to resistance while advancing toward the fluid chamber of the clutch so that the fluid pressure in the fluid chamber of the clutch increases at a limited rate. If the supply of the fluid pressure to the brake-release fluid chamber of the band servo unit is effected in an uncontrolled fashion, therefore, the fluid pressure to actuate the high-and-reverse clutch becomes lower than the fluid pressure to be developed in the brake-release fluid chamber of the band servo unit with the result that the clutch would be caused to complete its motion to couple after the brake band has been fully released. When this occurs, the engine tends to race during shifting from the second gear ratio to the third gear ratio in the automatic forward drive range. The present invention contemplates elimination of such a problem.